Martin (1992: 547):
The reason for separating field and genre as far as obligatory elements of text structure are concerned is to maintain inter-relationships among activity sequences and inter-relationships among text structures as distinct patterns of agnation regardless of mode …
Translating into SFL theory, this becomes:
The reason for separating the ideational dimension of context (field) and text type (genre),
as far as Hasan's obligatory elements of text structure are concerned,
is to maintain
inter-relationships among semantic structures viewed from below (Barthes)
and interrelationships between semantic structures viewed from above (Hasan)
as distinct patterns of agnation,
regardless of whether the rôle of language is ancillary or constitutive of the cultural context.
See the most recent posts for relevant critiques.
This all derives from Martin interpreting field as activity sequences and genres as social processes. A legitimate theoretical reason for separating field and genre is that field is context, not language, whereas genre is language, a type of text, not context.